In an FSC display apparatus, a color display is achieved by causing a plurality of colored lights (for example, of three primary colors, red, green, and blue) to be emitted in sequence and in a repetitive manner thereby displaying images corresponding to the respective colored lights.
One known example of such FSC display apparatus is an FSC liquid crystal color display apparatus which produces a color display by causing a plurality of colored lights (for example, of three primary colors, red, green, and blue) to be emitted in sequence and in a repetitive manner thereby displaying images corresponding to the respective colored lights on a liquid crystal display screen. A liquid crystal panel, such as a reflective liquid crystal panel, a transmissive liquid crystal panel, or a transflective liquid crystal panel, is used as the display screen of the FSC liquid crystal color display apparatus.
The above has described that the present invention can be applied to a liquid crystal display panel that uses a liquid crystal material as an electro-optical conversion material. The present invention can also be applied to an FSC display apparatus that uses a light control member or light control element such as described below, in which case the effect of the present invention can also be obtained.
Another example of the display apparatus is a display apparatus based on a technology called DLP (Digital Light Processing) developed by Texas Instruments, USA. This display apparatus has been on the market as a projection display apparatus. The core technology of the DLP is the DMD (Digital Micromirror Device) which is implemented on an optical semiconductor chip. The DMD comprises an array of 480,000 to 1,310,000 independently movable mirrors fabricated on a CMOS semiconductor chip. In the DLP, light from a lamp is projected onto the DMD and the light reflected by the mirrors of the DMD is projected through a lens for display.
The FSC display system is promising as a display system for a low-power liquid crystal display apparatus, because this display system does not use color filters and hence there is no loss of light associated with the use of color filters. However, a problem in an FSC display apparatus is the adjustment of color balance. Generally, in an FSC display apparatus, light-emitting diodes of three primary colors, red, green, and blue, are flashed in sequence and in a repetitive manner. However, because of differences in light-emitting efficiencies of the three color LEDs, it is not easy to obtain a correct color balance.
As one solution to this problem, there is proposed a technique that adjusts the color balance by changing the length of the subfield period for each color (for example, refer to patent document 1).
FIG. 14 is a diagram for explaining the technique that changes the length of the subfield period for each color.
In the technique shown in FIG. 14, one field period T is divided into three subfields, an R-subfield tR for red light emission, a G-subfield tG for green light emission, and a B-subfield tB for blue light emission, and the color balance is adjusted by changing the relative length of each subfield period. Here, the field frequency of one field is 60 Hz, and one field period T is therefore 16.67 ms.
In the technique shown in FIG. 14, the 16.67 ms period is not divided into three equal subfields, but is divided in such a manner that when the B-subfield period tB is made longer, the R-subfield period and the G-subfield period are made correspondingly shorter to adjust the color balance. That is, the color balance is adjusted by changing the relative length of the emission time of each color.
Such a color balance adjustment is possible because the FSC display apparatus produces a display by utilizing the accumulated response of the human eye.
Generally, the response speed of a liquid crystal display is slow and, because of this, the liquid crystal display has the problem that, if a light source is activated during the period that display data of a given pixel is being written (or immediately after the data is written), color uniformity or the color balance relative to other colors is deteriorated because a display is produced only matches the partially changed state of the liquid crystal.
To avoid this problem, there is proposed a technique in which, after the display data of each pixel has been written in each subfield, a prescribed time is allowed for the liquid crystal to respond, after which the light source is activated, thus preventing the color produced in one subfield from mixing into the color produced in another subfield (for example, refer to patent document 2).
FIG. 15 is a diagram for explaining the technique for activating the light source after waiting until the liquid crystal responds.
In the technique shown in FIG. 15, one field period T is divided into three subfields, an R-subfield tR for red light emission, a G-subfield tG for green light emission, and a B-subfield tB for blue light emission. The R-subfield tR is further divided into a writting period twr, a response period twa, and a light-emission period twi. Likewise, each of the G-subfield tG and the B-subfield tB is also divided into three periods.
In the R-subfield tR, data to be displayed in red on the liquid crystal display is written during the writting period twr, which is followed by the response period twa which allows a time for each pixel of the liquid crystal to change state in response to the thus written data, and the red light is emitted in the light-emission period twi. In this way, the red light is not emitted during the writting period twr or the response period twa. That is, to avoid the problem that color uniformity or the color balance relative to other colors is deteriorated, the light source is not activated until each pixel of the liquid crystal changes to the state that matches the red display data.
In the technique shown in FIG. 15, each subfield begins in synchronism with the beginning of the writting period of the corresponding color.
The technique shown in FIG. 15 also involves the following problem. The problem is associated with the slow response of the liquid crystal device.
When a TN or STN mode liquid crystal device is used for the liquid crystal display apparatus, if the response speed is increased by reducing the cell gap, it still takes about 2 ms until the transmittance characteristics of the liquid crystal device stabilize. Further, it takes about 1 ms to write data to be displayed on the liquid crystal display apparatus, even in the case of a small-screen QVGA-class display panel of 320 dots×240 dots. As earlier described, one field period T is generally about 16.67 ms, but the length of the field period cannot be made longer than that because of the problem of flicker. Therefore, the average length of one subfield taken over the three subfields into which one field is divided is about 5.56 ms or less. As shown in FIG. 15, when the 1 ms time required to write data to the liquid crystal display device and the 2 ms time allowed for the transmittance characteristics of the liquid crystal device to stabilize are subtracted from the 5.56 ms time, the time left for flashing the light source is only about 2.56 ms. Such a short light-emission flashing period leads to the problem that the display brightness decreases when a conventional light source is used.
One method to avoid this problem is to lengthen the light-emission period by shortening the liquid crystal response wait period, for example, to about 1 ms. While this method serves to retain the brightness of the liquid crystal display, there still remains the problem of color nonuniformity, or brightness nonuniformity, because sufficient time is not allowed for the transmittance characteristics of the liquid crystal device to stabilize.
The patent document 2 further describes that the color balance is adjusted by varying the ON time of the light source. To describe with reference to FIG. 15, in the method described in the patent document 2, the rise timing of the light-emission period twi is changed, but the fall timing is made to coincide with the end timing of the subfield. This method is simple in terms of the control circuit configuration.
However, when the color balance is adjusted by changing the rise timing of the light-emission period twi, as the transmittance characteristics of the liquid crystal device are not stabilized by the time when the light-emission period twi rises, there arises the problem that the amount of adjustment differs depending on the position on the liquid crystal display screen and that the liquid crystal is affected by variations in temperature. The reason for this will be described with reference to FIG. 16.
FIG. 16 is a diagram showing the transient response of the transmittance rate of the liquid crystal device.
In FIG. 16, the ordinate represents the transmittance rate S, and the abscissa represents the time T elapsed from the time that the writing of data to the liquid crystal display is started. It is assumed here that the liquid crystal display comprises a plurality of liquid crystal elements which are selected and sequentially driven as the respective scan lines are scanned. In FIG. 16, a curve 40 shows the characteristic of the first selected liquid crystal element at normal temperature, a curve 42 shows the characteristic of the first selected liquid crystal element at low temperature, and a curve 44 shows the characteristic of the first selected liquid crystal element at high temperature. Likewise, a curve 41 shows the characteristic of the last selected liquid crystal element at normal temperature, a curve 43 shows the characteristic of the last selected liquid crystal element at low temperature, and a curve 45 shows the characteristic of the last selected liquid crystal element at high temperature.
At normal temperature, the transmittance of the first selected liquid crystal element (see the curve 40) begins to rise when time t0 has elapsed from the beginning of the data writting (T=0). When time t1 has elapsed, the transmittance rate of the first selected liquid crystal element reaches about ½ of its stable state and, when time t1+t2 has elapsed, it reaches a substantially stable state. At low temperature, the first selected liquid crystal element exhibits a characteristic somewhat delayed in time (see the curve 42) compared with the characteristic at normal temperature and, at high temperature, it exhibits a characteristic somewhat advanced in time (see the curve 44) compared with the characteristic at normal temperature.
Data writing to the last selected liquid crystal element is started with a delay of time t1 with respect to the first selected liquid crystal element. Accordingly, the transmittance rate of the last selected liquid crystal element (see the curve 41) begins to rise when time t0 has elapsed from the beginning of the data writting (T=t1). When time t1+t2 has elapsed, the transmittance rate of the last selected liquid crystal element reaches about ½ of its stable state and, when time t1+t2+t3 has elapsed, it reaches a substantially stable state. At low temperature, the last selected liquid crystal element exhibits a characteristic somewhat delayed in time (see the curve 43) compared with the characteristic at normal temperature and, at high temperature, it exhibits a characteristic somewhat advanced in time (see the curve 45) compared with the characteristic at normal temperature.
As earlier noted, if the light source is flashed after the entire liquid crystal display has stabilized (over the entire operating temperature range of the liquid crystal display), there arises the problem that the light-emission period becomes extremely short and the brightness of the liquid crystal display decreases.
Here, one possible method to increase the brightness of the liquid crystal display is to lengthen the light-emission period twi, for example, by setting the writing period twr in FIG. 15 approximately equal to t1 in FIG. 16 and the response wait time twa in FIG. 15 approximately equal to t2 in FIG. 16. Then, the rise timing of the light-emission period twi occurs at time t1+t2 in FIG. 16. By this time, the transmittance rate of the first selected liquid crystal element has reached a substantially stable state (see the curves 40, 42, and 44), but the transmittance rate of the last selected liquid crystal element has reached only ½ of its stable state (see the curves 41, 43, and 45).
As a result, if it is attempted to adjust the color balance by changing the rise timing of the light-emission period twi in the vicinity of the time t1+t2 (i.e., by adjusting the length of the light-emission period for each color), there is an appreciable difference between the transmittance rate state of the first selected liquid crystal element and the transmittance rate state of the last selected liquid crystal element. That is, it is extremely difficult to achieve the color balance over the entire liquid crystal display screen.
Furthermore, if the color balance of the liquid crystal display is adjusted prior to shipment from the factory, there arises the problem that the color balance is deteriorated due to changes in temperature in the operating environment, because the transmittance characteristics of the liquid crystal elements greatly depend on the temperature (see FIG. 16).
In the case of fast-response liquid crystals such as ferroelectric or antiferroelectric liquid crystals, the response wait period twa shown in FIG. 15 is not provided or is set extremely short, and the light-emission period twi is lengthened, to increase the brightness of the liquid crystal display. Here also, there arises not only the problem that, if it is attempted to adjust the color balance by changing the rise timing of the light-emission period twi, it is extremely difficult to achieve the color balance but also the problem that the color balance is deteriorated due to changes in temperature in the operating environment.
In the patent document 2, it is pointed out that even when the light source for the preceding subfield is ON, the writing of display data for the next color can be started without any problem as long as the writing is started within the period during which the liquid crystal does not respond.
Patent document 1: Japanese Unexamined Patent Publication No. H09-274471
Patent document 2: Japanese Unexamined Patent Publication No. 2000-214435